Essentially it is permission for a Midshipman to leave the Yard (campus.)
Assuming your asking as/for a prospective Mid, so in that case, town liberty as it relates to Plebes is usually just permission to leave the Yard on Saturday. Timing can vary, especially during football season when town liberty starts immediately following a home game's conclusion.
And at the risk of stating the obvious, in the midst of a pandemic, all bets off.
From the USNA website:
Town Liberty – Town liberty will commence at the discretion of the company commanders. Company commanders will publish their requirements for each class to earn town liberty during the week. Normally, liberty for plebes will be authorized on Saturday from 1200 to 2400 (12:00 noon to 12:00 a.m. midnight) and Sunday for religious services only from 0800 to 1300 (8:00 A.M. to 1:00 P.M.) or begin after the last military obligation is completed, whichever is later. Plebes shall remain within the tri-city metropolitan area (Baltimore, Washington, D.C., Annapolis) during town liberty.